Alcohol dehydrogenases are a group of dehydrogenase enzymes that occur in many organisms and facilitate the conversion between alcohols and aldehydes or ketones. In humans and many other animals, they serve to break down alcohols which could otherwise be toxic; in yeast and many bacteria they catalyze the opposite reaction as part of fermentation.

The EC number of alcohol dehydrogenases is EC 1.1.1.1; their CAS number is 9031-72-5.

In humans

In humans, the enzyme is contained in the lining of the stomach and in the liver. It catalyzes the oxidation of ethanol to acetaldehyde:

CH₃CH₂OH + NAD+ → CH₃CHO + NADH + H⁺

This allows the consumption of alcoholic beverages, but its original purpose is probably the breakdown of alcohols naturally contained in foods or produced by bacteria in the digestive tract. Note that acetaldehyde is even more toxic than ethanol and is responsible for many of the hangover symptoms, but it is quickly converted to acetic acid and other harmless molecules.

Alcohol dehydrogenase also oxidizes methanol to formaldehyde, and the dangerous effects of the latter are the main reason for the toxicity of the former. The conventional treatment of methanol poisoning involves administration of ethanol: this keeps the alcohol dehydrogenase busy with ethanol, so that the methanol can be cleared out of the system before it is converted to formaldehyde. There is also now a newer drug for treating methanol poisoning: it blocks the action of alcohol dehydrogenase.

Humans have at least six slightly different alcohol dehydrogenases. All of them are dimers (consist of two polypeptides), with each dimer containing two zinc ions Zn²⁺. One of those ions is crucial for the operation of the enzyme: it is located at the catalytic site and holds the hydroxyl group of the alcohol in place.

The version of alcohol dehydrogenase in women is less effective than that in men, which is part of the reason that women have a lower tolerance for alcohol than men.

In yeast and bacteria

In yeast and many bacteria, alcohol dehydrogenase plays an important part in fermentation: pyruvate resulting from glycolysis is converted to acetaldehyde and carbon dioxide, and the acetaldehyde is then reduced to ethanol by alcohol dehydrogenase. The purpose of this latter step is the regeneration of NAD+, so that the energy generating glycolysis can continue. Humans exploit this process to produce alcoholic beverages, by letting yeast ferment various fruits or grains.

The main alcohol dehydrogenase in yeast is larger than the human one, consisting of four rather than just two subunits. It also contains zinc at its catalytic site. It is clear that the human and yeast alcohol dehydrogenases are closely related.

Insects

In insects such as the fruit fly, the alcohol dehydrogenase is smaller than in humans, does not contain a metal, and appears to be unrelated.

Iron containing

A third class of alcohol dehydrogenases, unrelated to the above two, are iron containing ones. They occur in bacteria, and an (apparently inactive) form has also been found in yeast.

External links

• PDBsum has links to three-dimensional structures of various alcohol dehydrogenases contained in the Protein Data Bank
• ExPASy contains links to the alcohol dehydrogenase sequences in Swiss-Prot, to a Medline literature search about the enzyme, and to entries in other databases.
• BRENDA most comprehensive compilation of information and literature references about the enzyme; requires payment for commercial users